Inductive power transfer (IPT) systems provide one example of wireless transfer of energy. In IPT systems, a primary power device (or “transmitter”) transmits power wirelessly to a secondary power device (or “receiver”). Each of the transmitter and receiver includes an inductive coupler, typically a single or multi-coil arrangement of windings comprising electric current conveying materials, such as Litz wire. An alternating current passing through a primary coupler produces an alternating magnetic field. When a secondary coupler is placed in proximity to the primary coupler, the alternating magnetic field induces an electromotive force (EMF) in the secondary coupler according to Faraday's law, thereby wirelessly transferring power to the receiver.
In order to operate with high efficiency, IPT systems should be capable of switching to a positive voltage phase or to a negative voltage phase when the driving alternating currents are as near as possible to a zero crossing point (“soft switching”). Under symmetric switching conditions, this may be the equivalent of the IPT system operating as near as possible to a unity power factor, where the current and voltage waveforms are substantially in phase with one another. However, because the inductance of the IPT transmitter may vary significantly depending on coupling with an IPT receiver, such unity power factor is not generally achievable utilizing symmetric duty cycle positive and negative voltage phases. This can result in switching between positive and negative voltage phases of the driving voltage waveform when the IPT driving current is at significant non-zero values (“hard switching”). This can cause excessive power dissipation in or failure of the IPT driver switches. As such, systems, methods and apparatuses implementing hybrid symmetric and asymmetric control for soft switching in wireless power transfer applications are desirable.